Tanks used for storing or transporting flammable fluids such as gasoline, diesel fuel and other hazardous petroleum products are often equipped with overfill protection devices. When the fluids are being transferred from the storage tanks to recipient tanks, these devices detect when the recipient tanks are full and automatically disable the transfer process. Thus, waste is avoided and the environment is protected from spillage due to overfilling.
Tanks can be mounted on tanker trucks or located underground at service stations. Tanker trucks are typically filled with the fluids using pumping equipment at the loading racks of marketing terminals, and underground storage tanks are typically gravity-filled from the trucks. An overfill protection device is used with each tank to disable the pumping equipment at the marketing terminals or to close a truck-mounted flow valve at the service station when the limit of the recipient tank's capacity is reached.
Typically, an overfill protection device has a detection circuit and a disable circuit. The detection circuit has a sensor located within the tank which generates an electrical sensor signal indicating when the fluid in the tank exceeds a pre-determined level. The detection circuit is connected to the disable circuit by an electrical cable used to communicate the sensor signal to the disable circuit. When the signal indicates that the fluid level in the recipient tank has reached a desired capacity, the disable circuit responds by stopping the fluid flow into the tank (by disabling the pumping equipment at the loading rack or by closing the flow valve on the truck).
One embodiment of the detection circuit generates a pulsed signal for use with a sensor which converts it into an optical pulse signal that is reflected from a prism in a storage tank to a light detector when the fluid level is below a desired capacity. In response, the light detector generates a pulsed signal which is returned to the detector circuit. When the fluid level in the tank reaches the desired capacity, the indices of a refraction between the prism and the surrounding fluid prevent reflection of the optical pulse signal to the light detector. The light detector thus ceases to generate the pulsed signal and the system responds by disabling fluid flow into the tank, thus preventing an overfill condition.
Although such level sensing sensors are useful for preventing the overfill of recipient tanks, they are unable to identify the type of fluid being introduced into the tank. Thus, on occasion, situations arise where a particular fluid is accidentally introduced into a tank intended for a fluid of a different grade or a different fluid altogether. This usually results in the fluid introduced mixing with the residual fluid in the tank sometimes rendering it useless. Such mixing of products may also cause damage or create a safety hazard. In addition, the mixed fluid often must be discarded, creating a difficult and expensive disposal job.